Copper is a metal excellent in electric conductivity but inferior in mechanical strength. For the reason, in practical uses, it is a customary countermeasure to reinforce copper by the addition of some additive (an alloying element or elements). However, in the fields where conductivity is of prime importance (e.g., in the manufacture of very thin wires such as magnet wires, lead wires for electronic components, lead members such as TAB or others, and members for printed-circuit boards), pure copper (with purity on the order of 99.99%) is usually used to keep the outstanding conductivity of copper unimpaired.
A problem is that the higher the purity the softer copper becomes, with the increasing risk of breaking due to "stretching-to-break" during wire drawing or other similar working operation. In particular, it has been pointed out in the art that coating with urethane, polyimide, or the like, lessens seriously the mechanical strength of pure copper wires or members or parts, rendering it difficult for those to maintain their shapes, which causes "bending or turning", "over-elongation", "droop", or other troubles. Nevertheless, "high electric conductivity" has remained the most required of the properties of conductive materials for electric wires and other similar applications. Since conductivity is preferred to mechanical strength (which is intimately related to wire drawability and other working characteristics of the material), heat resistance, and other properties, pure copper has predominantly been obliged to be used.
Meanwhile, there have been intensified demands over the years for the miniaturization of electronic components, for thinner electric wires, and for efficient operation of the manufacturing processes. To keep up with the trend, requirements for copper materials are becoming more and more exacting. Materials not merely possessing excellent conductivity but, in addition, combining conductivity with greater mechanical strength, heat resistance, and other properties are in stronger demand than heretofore.
In view of these, the present applicant, in its attempts at meeting the above requirements, previously made some proposals as to "copper materials based on high-purity copper with the addition of minute amounts of In, Hf, Mg, Be, B, Zr, Y, Ag, Si, Ca, or/and a rare earth element or elements" (Patent Application Public Disclosure Nos. 127436/ 1987 and 127438/1987, and Patent Application No. 73152/1988).
The above copper materials proposed by the applicant exhibited better mechanical strength and heat resistance than conventional products while retaining the conductivity of the level of pure copper. Those favorable properties promised a high contribution of the materials to the qualitative improvement in electric and electronic components.
However, the prospects of ever escalating performance requirements are suggesting that there are still limits to such materials in the points of mechanical strength, heat resistance, and other properties.